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Aromatic Hydrocarbons

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  • Last Updated : 21 Sep, 2021
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In chemistry, we have learnt about alkanes, alkenes and alkynes. These all are linear in nature. Apart from these hydrocarbons, we also have cyclic compounds. Aromatic hydrocarbons are hydrocarbons and their alkyl, alkenyl, and alkynyl derivatives that include one or more benzene ring fused or isolated in their molecules. The benzene ring is extremely unsaturated, and the unsaturation of the benzene ring is preserved in most aromatic compound reactions. Aromatic hydrocarbons with a benzene ring are known as benzenoids, whereas those without a benzene ring are known as non-benzenoids. Let us see what are cyclic compounds and later learn about aromatic compounds. 

What are Cyclic Compounds?

A compound in which the atoms are linked together to form a ring or loop-like structure is termed a cyclic compound. They can be fully saturated or unsaturated i.e. they may contain only sigma bonds or both sigma and pi bonds. 

Cyclic compounds are further of two types:

  1. Carbocycles: Compounds in which all the atoms are Carbon
  2. Heterocyclic compounds: Compounds that have both Carbon and non Carbon atoms.

Examples of cyclic compounds are cyclopropane, cyclobutane, etc.

Cyclopropane and cyclobutane

Now that we have learnt about cyclic compounds, we shall study aromatic compounds.

Aromatic Compounds

Cyclic Compounds that have planar rings with completely delocalized pi electrons are called aromatic compounds. They are also known as arenes. Aromatic compounds have a unique aroma and are hence termed aromatic compounds. 

e.g.: Phenol, naphthalene, etc.

Phenol

In the above image, we can see that the ring has completely delocalized pi electrons. Aromatic compounds can be classified as benzenoids and non-benzenoids depending upon whether they contain a benzene ring or not.

Properties of Aromatic Compounds are:

  1. They are non-polar in nature
  2. They are immiscible in water due to their non-polar nature
  3. They burn with a sooty yellow flame due to the high Carbon- Hydrogen ratio
  4. They are used as solvents for non-polar compounds and are usually unreactive.
  5. They undergo electrophilic substitutions and nucleophilic aromatic substitution reactions.

It is very difficult to identify an aromatic compound just by looking at its structure. Thus we make use of the Huckel rule. So the definition of aromatic compounds can be extended as compounds that follow the Huckel rule are known as aromatic compounds and the property is known as aromaticity.

Huckel’s Rule

According to Huckel’s rule, the aromatic compounds may possess the following properties or satisfy the following criteria:

  1. Planarity: The compound must be planar i.e. it should not have any Carbon-Carbon single bonds. All the Carbon atoms in the compounds must be sp^2 hybridized.
  2. Delocalized pi electrons: The compound must have a completely delocalized \pi electron cloud both above and below the ring.
  3. 4n+2 pi electrons: This is the most important condition of Huckel’s rule. According to it, an aromatic compound must have 4n+2 \pi electrons where n is an integer and n = 0, 1, 2, 3, 4,….

Let us look at an example to verify Huckel’s rule. Consider the compound shown below i.e. benzene

The above compound is planar in nature as all the Carbon atoms are sp2 hybridized. The compound also has completely delocalized pi-electrons. Now, the no. of \pi electrons in benzene is 6 which is equal to 4(1)+2 for n = 1. Thus it satisfies all the conditions of Huckel’s rule and is, therefore, an aromatic compound.

Anti Aromatic Compounds

Apart from aromatic compounds, there are antiaromatic compounds also. Anti aromatic compounds follow the first two conditions of Huckel’s rule but differ in the third one. They have (4n\π) electrons instead of (4n+2) \ π electrons. These compounds are very reactive in nature and highly unstable.

e.g.: Pentalene with 8πe.

Uses of Aromatic Compounds are:

  • The nucleic acids and amino acids present in the human body are aromatic in nature
  • The chlorophyll present in plants is also an aromatic compound and is necessary for the survival of plants and make food.
  • Naphthalene which is an aromatic compound is used to make mothballs to protect clothes.
  • Tri Nitro Toluene is used to make explosives
  • They are used to make drugs and dyes
  • They are used extensively in the plastics and petrochemical industry.

Sample Questions

Question 1: Define aromatic compounds.

Answer:

Cyclic Compounds that have planar rings with completely delocalized pi electrons are called aromatic compounds. They are also known as arenes. Example: Benzene, Naphthalene

Question 2: Define antiaromatic compounds.

Answer:

Compounds that are planar in nature and have completely delocalized pi electrons but have 4n\pi electrons are called anti aromatic compounds.

Question 3: State two differences between aromatic and antiaromatic compounds.

Answer:

The difference between aromatic and anti aromatic compounds are as follows:

Aromatic

Anti Aromatic

They have 4n+2 pi electrons They have 4n pi electrons
They are very stable and unreactive They are highly reactive in nature

Question 4: State Huckel’s rule.

Answer:

According to Huckel’s rule, the aromatic compounds may possess the following properties or satisfy the following criteria:

  1. Planarity: The compound must be planar i.e. it should not have any Carbon-Carbon single bonds. All the Carbon atoms in the compounds must be sp^2 hybridized.
  2. Delocalized pi electrons: The compound must have a completely delocalized \pi electron cloud both above and below the ring.
  3. 4n+2 \pi electrons: This is the most important condition of the Huckel’s rule. According to it, an aromatic compound must have 4n+2 \pi electrons where n is an integer and n = 0, 1, 2, 3, 4,………

Question 5: State two uses of aromatic compounds.

Answer:

Following are the two uses of aromatic compounds are:

  • Naphthalene which is an aromatic compound is used to make mothballs to protect clothes.
  • Tri Nitro Toluene, an aromatic compound, is used to make explosives.

Question 6: State any two properties of aromatic compounds.

Answer:

Following are the two properties of aromatic compounds are:

  • They are immiscible in water due to their non polar nature.
  • They burn with a sooty yellow flame due to high Carbon-Hydrogen ratio.

Question 7: Verify if the given compound is aromatic or not.

Answer:

The compound shown in above image is planar in nature as all the Carbon atoms are sp2 hybridized but it has 4 pi electrons which violates Huckel’s rule. Hence it is not aromatic but anti aromatic.

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